When cell selection, reselection, or handover is performed, user equipment (UE) needs to perform cell synchronization and identification, channel state information (CSI) measurement, and radio resource management (RRM) measurement according to a reference signal that is sent by a base station. The cell synchronization includes initial coarse synchronization and time-frequency track fine synchronization. Specifically, the UE may complete the initial coarse synchronization according to a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) that are periodically sent by the base station, and complete the time-frequency track fine synchronization according to a cell-specific reference signal (CRS) that is periodically sent by the base station. The CSI measurement includes channel measurement and interference measurement. The UE may perform the channel measurement and the interference measurement according to the CRS or a channel state information-reference signal (CSI-RS) that is sent by the base station. The RRM measurement includes reference signal received power (RSRP) measurement, reference signal received quality (RSRQ) measurement, and received signal strength indicator (RSSI) measurement, and the like. The UE may complete the RRM measurement according to the CRS that is periodically sent by the base station.
However, in a communications system that uses an unlicensed spectrum, when communication is performed by using the unlicensed spectrum, a specific coexistence specification needs to be met, for example, the Listen-Before-Talk (LBT) specification. Specifically, before sending a signal on a channel that is corresponding to the unlicensed spectrum, the base station needs to perform clear channel assessment (CCA) on the channel. If detected receive power exceeds a preset threshold, it is considered that the channel is busy, and the base station cannot send a signal on the channel in this case. The base station can send a signal on the channel only when it is detected that the channel is idle.
Therefore, due to limitations of the LBT specification, a periodic sending feature of reference signals such as the PSS, the SSS, the CRS, and the CSI-RS is affected. For example, at a sending opportunity within a preset period, the base station may detect that the channel is busy, and consequently cannot send a reference signal at one or more sending opportunities. This affects cell synchronization and CSI measurement that are performed by UE, and further affects mobility performance of the UE. In this case, how to send a reference signal in the communications system that uses an unlicensed spectrum, to meet requirements for cell synchronization and CSI measurement is a problem to be resolved urgently.